1. Field of the Invention
The present invention relates to an interferometer and to a shape measuring device using an interferometer. The present invention provides a device for measuring the surface shape of a relatively-large diameter lens, mirror, die, etc. used in a camera, a video, a semiconductor manufacturing apparatus or the like. The device is particularly suitable for the measurement of an aspheric shape which is hard to measure with an ordinary interferometer.
2. Description of the Related Art
As conventional examples of the art, two measuring devices disclosed in K. Yoshizumi et al., Precise Measuring System for Aspheric Surfaces, Optics, vol. 12, No. 6, pp 450-454 (December, 1983) will be described.
FIG. 1 shows the construction of the first conventional example.
In the drawing, numeral 901 indicates a Zeeman laser constituting the light source; numeral 902 indicates a beam splitter; numerals 903 and 904 indicate polarizing beam splitters; numerals 905a and 905b indicate .lambda./4 plates; numeral 906 indicates an objective lens; numeral 907 indicates a reference surface; numeral 908 indicates a workpiece; numeral 909 indicates a workpiece stage; numeral 910 indicates a focus detector; numerals 911a and 911b indicate beat signal detectors.
Assume two beams which are emitted from the light source Zeeman laser 901, which are polarized in directions perpendicular to each other and which have slightly different frequencies f1 and f2. These two beams are spatially separated from each other by the first polarizing beam splitter 903. The beam f1 travels straight and is transmitted through the second polarizing beam splitter 904 to be converted to circularly polarized light by the .lambda./4 plate 905a before it is applied to the surface of the object of measurement (workpiece) 908 on the workpiece stage 909 so as to focus thereon by the objective lens 906. Then, it returns to the objective lens 906 due to the so-called Cat's Eye reflection. It is transmitted through the the .lambda./4 plate 905a again to be converted to linear polarized light before it impinges upon the second polarizing beam splitter 904 in a condition in which it has been rotated by 90.degree. from the linear polarized light before the reflection. This polarizing beam splitter 904, with a special coating, divides the returned beam into two portions, one of which is led to the first polarizing beam splitter 903 and the other of which is reflected and led to the focus detector 910.
In this device, the objective lens 906 is servo-controlled in the optical-axis direction by using a signal of the focus detector 910 so that focusing may always be effected on the workpiece surface even when the workpiece moves in a direction perpendicular to the optical axis.
On the other hand, the light beam reflected by the polarizing beam splitter 903 is converted to circularly polarized light by the .lambda./4 plate 905b, and reflected by the reference surface 907 arranged on the workpiece stage 909 before returning to the polarizing beam splitter 903. Since it passes through the .lambda./4 plate 905b again, the beam is in the form of a linear polarized light with the direction of polarization rotated by 90.degree.. The beam is transmitted through the polarizing beam splitter 903 and directed to the beat signal detector 911b.
The beam which has been reflected by the workpiece and returned to the polarizing beam splitter 903 is also directed to the beat signal detector 911b, so that interference occurs with the light reflected by the reference surface, and a measurement beat signal is detected by the beat signal detector 911b.
This measurement beat signal is caused to interfere with the light immediately after emission from the light source to measure the difference in phase between the signal and a reference beat signal obtained by the beat signal detector 911a, and the difference in phase when scanning is performed on the workpiece in a direction perpendicular to the optical axis is integrated, whereby the shape of the workpiece is measured.
In the second conventional example shown in FIG. 2, a light beam whose diameter is smaller than the effective diameter of the objective lens 906 is used. The axis of the objective lens is also movable in a direction perpendicular to the optical axis, and the focus detector 910 is endowed with a surface inclination detecting function, a system being added by means of which servo-control is effected such that the light beam always impinges at right angles in correspondence with the surface inclination of the workpiece.
The first conventional example, described above, has a problem in that, when the surface to be measured is inclined, the reflected light is eclipsed, so that the quantity of light returned to the detector varies to an extreme degree, thereby making the measurement difficult. In particular, in the case of the measurement of an aspheric surface or the like, a more accurate measurement cannot be expected.
In the second conventional example, the problem of the first conventional example, i.e., the variation in quantity of light when the surface to be measured is inclined, can be avoided. However, due to the addition of the two axes (the X and Y-directions) to the servo system, the device is further complicated.